The feds may claim that it’s just “upgrades and replacement of utility infrastructure” of a size and complexity that require the excavation of a hole so big it could easily fit the entire Cabinet — and maybe some members of Congress, too. But they’re somewhat vague as to the specifics. And so we wonder.

It’s a bunker, right? It’s gotta be a bunker.

Make that another bunker. There’s already the underground Situation Room, which was renovated and expanded to 5,000 square feet in 2007, and the nuclear bomb shelter below the East Wing, where Vice President Richard B. Cheney was taken on the morning of Sept. 11, 2001. And who knows what else sits beneath the White House? A tunnel to the State Department? A secret passage to the Treasury?

With all that subterranean space, think of the White House not just as a 200-year-old, neoclassical Federal-style mansion, but as the tip of an iceberg. Think of what you don’t see. It’s a building with roots, which in D.C. is not uncommon.

Underground is where it’s at in Washington — and not only for the president, who makes some of his most important decisions while at the altitude of a mole. Thanks to a height restriction that keeps the skyline in a sort of humble genuflection to the Washington National Cathedral, the city’s highest structure, there are no 100th-floor views. The elevators barely make it to double digits. From the rooftops, people on the streets below look like dolls, not ants.

No matter. In Washington, depth, not height, is the measurement that matters.

This is metaphorically true — information and knowledge, not money and glitz, are what really equate to power here — but it’s also literally true.

Much of Washington is down and out of sight. Some of this world we know well and visit often, such as Metro or the passageways that connect the Capitol’s buildings. Much of the Smithsonian’s Sackler Gallery is underground. The National Aquarium is beneath the Commerce Department building. There are steam tunnels and old Civil War bunkers and deep-down parking garages.

The height restriction “puts out of sight stuff that doesn’t need to be on the street — like lots of parking garages, which are deadly to the urban experience,” said Thomas Luebke, the secretary of the U.S. Commission of Fine Arts, the design review agency that oversees development on the Mall.

Imagine 200 years from now, a post-apocalyptic Washington, he says, “abandoned and left to collapse.” Imagine everything gone, the Capitol, the Washington Monument, all the buildings downtown. What would be left?

“You would find this landscape of elevated streets surrounded by pits,” he says.

Like the Ardennes forest, which is still pocked with World War II bunkers, the pits would endure.

* * *

And the pits have gone upscale.

Come, enter the underground Capitol Visitor Center. Follow the walkway, which tilts down so slightly that you barely notice you’re being led below. Inside, the floors gleam, light pours in through the skylights.

It’s airy and spacious — three-quarters the size of the Capitol itself. During construction, crews hauled away 65,000 truckloads of soil to create this cavernous space. It may be underground, but even claustrophobics wouldn’t feel cramped.

It’s easy to forget you’re below grade until you follow the tour guide up the escalator into the Capitol and notice through the windows that you’re now emerging to the surface.

That’s the feel the designers of the visitors center at the Vietnam Veterans Memorial are aiming for. They, too, are planning to build down. At first they wanted to build up — expand the wall’s kiosk with a small structure no more than 1,500 square feet. But real estate on the Mall is heavily restricted, and even such a modest building was frowned upon.

Underground they could build out to 30,000 square feet or more, though. Instead of an expanded kiosk, they started thinking museum.

Beneath the soil was the Western frontier circa 1850. Manifest Destiny pointed down, not west.

It also would keep the visitors center from intruding on the rest of the Mall, which is getting pretty full. Everything that’s added now, people fear, will impede on everything that’s there already.

“The plan for the city was open space, wide boulevards,” says Dan Reese, of the Vietnam Memorial Fund. “You’ve got a restriction on height and a feeling of openness in this city that is really unmatched. To go underground makes good sense to preserve that.”

* * *

Then there’s the underground we’re not supposed to know about — or that is revealed years later. The Cold War bunker built under the Greenbrier resort in West Virginia is now a tourist attraction. Come see where the government was to be relocated in case Washington came under attack — the tour is only $30!

In 2001, when former FBI counterintelligence agent Robert P. Hanssen was arrested and charged with spying for the Russians, U.S. officials said he divulged many secrets. One of the most embarrassing was the alleged existence of a secret tunnel underneath the Russian Embassy in Glover Park used by the National Security Agency and the FBI for electronic eavesdropping.

Then, in 2002, neighbors near the Naval Observatory started complaining about daily construction blasts so loud they rattled windows and knocked mirrors off walls. What, the neighbors demanded to know, was going on at the vice president’s residence?

The answer they got was circumspect.

“Due to its sensitive nature in support of national security and homeland defense, project-specific information is classified and cannot be released,” the then-superintendent of the observatory wrote to the neighborhood advisory commissioner.

Officially, the Navy continued to maintain that the project comprised “infrastructure and utility upgrades.”

Utility upgrades. That sounds familiar.

The White House’s utility upgrade project began in May 2009, according to the General Services Administration. Crews are replacing aging electrical, cooling, heating and fire alarm equipment, a spokesman said.

But those who work in the White House aren’t buying it, either. One told the New York Times that the work is “security-related” and would ultimately create an expanded underground emergency operations center.

Whatever they’re doing outside the West Wing, the crews eventually will shift over to the north lawn by the East Wing and start digging there as well, the GSA says.

“Once the work nears completion, the grounds will be restored to their original state,” according to the GSA spokesman.

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